Mixing waters Marine life from the Pacific have crossed the Arctic Ocean in what may portend a marine invasion threatening Atlantic fish stocks, say scientists.

The Pacific algae (Neodenticula seminae), absent from the North Atlantic for 800,000 years according to fossil records, apparently returned after thawed sea ice and currents carried the microscopic plants across the Arctic Ocean, they say.

And a gray whale spotted in the Mediterranean in 2010 - 300 years after the species was hunted to extinction in the Atlantic region - is believed to have swum from the Pacific through newly ice-free waters in the Arctic Ocean in summertime.

"It's a Pandora's Box," says Professor Chris Reid, from the Sir Alister Hardy Foundation for Ocean Science in Britain who says the algae had now drifted almost as far south as New York. "We can expect more species to come through from the Pacific."

Upheavals to life in the seas have been documented by European scientists from 17 marine institutions in 10 nations in a project called CLAMER.

An influx of species could "be extremely damaging...for fisheries in the North Atlantic," says Reid. New arrivals would compete with established species such as cod or salmon.

One-way traffic

It would be far harder for species to go from the Atlantic to the Pacific, against currents and winds, the scientists say.

The gray whale, identified from photographs taken off Spain and Israel in May 2010 as the same individual, was also believed to have swum across the Arctic from the eastern North Pacific.

"We now have two signals of organisms passing this open water in the Arctic," says Katja Philippart of the Royal Netherlands Institute for Sea Research, which leads CLAMER.

Chances that gray whales had somehow survived undetected in the Atlantic region since 1700 - or that it had swum a far longer route such as around South America - were far less plausible, she says.

Similarly, the scientists say it was almost impossible that the algae could have arrived another way, for instance in ballast water of ships via the Panama Canal. The algae were cold-lovers and would have died on a route through the tropics.

Pacific organisms last crossed to the Atlantic some two million years ago, according to fossil records.

"Then there was a major invasion ... It completely changed the ecosystems of the North Atlantic," says Reid. The newly found algae had disappeared in the Atlantic about 800,000 years ago, coinciding with Ice Ages.

Among other shifts, CLAMER found that many species were moving north as waters warmed. "The number of fish species in the North Sea has increased from 60 to more than 80 in just 20 years," says Carlo Heip, director General of the Royal Netherlands Institute for Sea Research.

In the almost enclosed Mediterranean, species favouring cooler waters had nowhere to go. By 2060, CLAMER says a third of the sea's 75 species will be threatened and six will be extinct.